Embodiments of the invention relate generally to an MRI preamplifier and, more particularly, to an apparatus for improving the grounding of the preamplifier and method of making same.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In modern MRI phased array coils, the preamplifiers are placed near the receiving antennas. In general, the close proximity attempts to overcome fast signal-to-noise (SNR) degradation along the transmission lines and inherently weak signals coming from human body. Methods of reducing SNR degradation include amplifying the signal received by a receiving antenna by the preamplifier very close to the antenna output. Thus, a feedboard containing the preamplifier may be coupled directly to the antenna output. Moving the preamplifier close to the receiving antenna also tends to increase decoupling between receive elements.
However, moving the preamplifier close to the receiving antenna tends to make grounding of the preamplifier difficult to control. The preamplifier function is affected by its grounding. Hence, a good ground can positively affect the function of the amplifier, and a poor ground can negatively affect the function of the amplifier.
Generally, when the S12 of an empty feedboard is higher than the S12 of the stand-alone preamplifier such as, for example, what may occur when the S12 of the preamplifier is significantly low to have its μ-parameter higher than unity, the preamplifier's stability is affected. In high density coils, where RF components and cables are in close proximity to each other, the coil feedback (S12 or S21) between input and output of the preamplifier can reach high values that cause oscillations.
It would therefore be desirable to have an apparatus capable of stabilizing the grounding for MRI preamplifiers to reduce or eliminate the aforementioned drawbacks.